A driving system for a four-wheel drive vehicle includes: a part-time driving system for appropriately switching between four-wheel drive and two-wheel drive; a full-time driving system for continuously driving four wheels; and a standby driving system for transition between four-wheel drive and two-wheel drive states as occasion demands. In the standby driving system, a main drive wheel is directly coupled to an internal combustion engine, and a sub-drive wheel is connected to the internal combustion engine via a coupling. For the coupling, a distribution ratio of the driving force of the main drive wheel to that of the sub-drive wheel, that is, a torque distribution ratio is adjusted in accordance with the road surface condition or operating state. The coupling is controlled by a plurality of torque distribution modes including first and second torque distribution modes.
In the standby driving system, a detected value of a throttle sensor for detecting throttle opening of a throttle valve disposed in the internal combustion engine, and that of a rotation speed sensor, are compared with judgment threshold values. When each detected value is less than the corresponding judgment threshold value, the coupling is controlled in the first torque distribution mode. When each detected value is not less than the corresponding judgment threshold value, the coupling is controlled in the second torque distribution mode.
In the first torque distribution mode, the torque of the engine is supplied mainly to the main drive wheel. In the second torque distribution mode, as compared with the first torque distribution mode, the torque is nearly equally distributed to the main and sub-drive wheels. When the four-wheel drive vehicle travels around a curve, or when the driver steps on the accelerator, the torque distribution mode of the coupling is changed to the second torque distribution mode from the first torque distribution mode so as to enhance traction between the road surface and vehicle wheels. Moreover, when the four-wheel drive vehicle is operating in the second torque distribution mode, and the vehicle is not traveling around a curve, and the accelerator setting is not changed, the following happens. At this time, the torque distribution mode of the coupling is changed to the first torque distribution mode from the second torque distribution mode.
When the four-wheel drive vehicle travels on a mountain path or performs slalom-type operation, the first and second torque distribution modes are frequently switched. When the torque distribution mode is frequently switched in this manner, vehicle behavior change frequently occurs (switch shock), and ride quality is impaired.